The Russian newspaper that broke the news of the mass detention and torture of gay men in Chechnya has said it fears for the safety of its journalists after a televised gathering of religious and social leaders in Grozny promised “reprisals” against them.

Novaya Gazeta, one of the few remaining independent news sources in Russia, first reported on the anti-gay violence in Chechnya several weeks ago. Authorities have denied they are rounding up gay men, and dismissed the reports as lies.

Chechens tell of prison beatings and electric shocks in anti-gay purge: ‘They called us animals’

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The Guardian published testimony on Thursday from two Chechen men who had been caught up in the anti-gay operation, which backed up Novaya’s claims of a broad crackdown on gay men in the region. One of the men said he had been held in an informal prison for more than a week with other gay men, and subjected to repeated beatings and torture using electric shocks.

Rights organisations have called on the international community to put pressure on the Kremlin to ensure the repression in the southern Russian region is brought to an end. There are also calls for the numerous western film, music and sports stars who have made paid appearances with the region’s leader, Ramzan Kadyrov, to take a stand against the violence.

Chechen television reported that up to 15,000 people had gathered on Thursday at Grozny’s central mosque to pass a resolution against the “lies and libel” in the Novaya Gazeta stories – chiefly for suggesting there are gay men in Chechnya. A spokesman for Kadyrov has previously denied their existence, saying if there were gay people in Chechnya, their families would have killed them.

The resolution said: “The centuries-old traditions of Chechen society, the dignity of Chechen men, and our faith have all been insulted, and we promise that those behind it will face reprisals, whoever they are and wherever they are.”

Several Novaya journalists have been killed over the years, including Politkovskaya, who was gunned down in the entrance of her Moscow apartment block in 2006. She had written widely on rights abuses under Kadyrov.

The journalist who broke the story of the anti-gay repression, Elena Milashina, worked closely with Politkovskaya before her death and has continued to publish widely on Chechnya.

“My sources tell me there’s a serious danger, for me personally and for every employee of Novaya Gazeta,” said Milashina, who said she plans to take extra security precautions and possibly leave the country for a period.

“It’s clear that none of this has much to do with religion,” she said. “These official religious meetings are meant to act as a cover for professional killers; then they can blame it on offended religious feelings.”

Milashina and Muratov said the newspaper would not stop covering the story, and planned further publications on the subject.

The newspaper believes it has evidence that at least three people have been killed in the purge, and suspects the real figure could be much higher. Gay rights activists running a hotline to help gay men escape the region say they have received calls from “dozens” of men.

The foreign secretary, Boris Johnson, wrote on Twitter on Thursday that the reported crackdown was “outrageous”, but the Liberal Democrat leader, Tim Farron, criticised him for cancelling a trip to Russia earlier this week, saying it would have been an opportunity to put pressure on the Kremlin in person.

“The UK government must strongly condemn this disgusting violence, and use every possible opportunity to raise this with the Russian government,” Farron told the Guardian in comments emailed by a spokesperson.

“It is disappointing that Boris Johnson has failed to stand up for these people by not visiting Russia after Trump told him to stay at home.”

Human rights campaigner Peter Tatchell was one of many who called on western celebrities who have controversially accepted Kadyrov’s invitations in the past to raise their voice over the recent crackdown.

“I would urge celebrities who have been to Chechnya to speak out against the current anti-gay witch-hunt and to use their personal contacts in Chechnya to pressure the government to halt the arrests, torture and killing,” he told the Guardian.

Many western celebrities have travelled to Grozny to take part in Kadyrov’s publicity projects, often for money. They include the actors Gérard Depardieu, Hilary Swank, Jean-Claude Van Damme and Liz Hurley, the footballers Robbie Fowler, Steve McManaman and Diego Maradona, and the musicians Vanessa Mae and Seal.

Björn van Roozendaal, programme director at ILGA Europe, said celebrities who have been friendly with Kadyrov have a moral obligation to speak out about the recent wave of violence against the region’s LGBT community.

“The sad reality is that the leverage that the international community has with Chechnya and with Russia at the moment is limited so the pressure from other parts of the society that have an influence are extremely valuable,” he told the Guardian.

“It is extremely important that anyone who has an influence speaks out, it’s people’s moral duty at the moment.”

Of all the celebrities who have visited Chechnya over the years, Swank is the only one to publicly apologise for her appearance with Kadyrov. She claimed to have been misinformed about the republic and, after a media scandal, fired her manager and donated her appearance fee to charity.